code: def __len__(self):
      return 10
  
imports: import numpy
  import unittest
  from chainer.dataset import DatasetMixin
  from chainercv.utils import assert_is_bbox_dataset
  from chainercv.utils import generate_random_bbox
  from chainercv.utils import testing
label_desc: Use image node_id 2 for calling the BboxDataset obj's underlying member method code with example usage: obj.__len__() and returns: int